Southern Uplands geology: an historical perspective

Abstract

ABSTRACTFor more than two centuries the rocks of the Southern Uplands have provided successive generations of geologists with both illumination and frustration. This area furnished vital evidence to support Hutton's ideas of petrogenesis and his cyclical view of earth history. However, for much of the succeeding century construction of a credible and consistent stratigraphy and structure eluded the best efforts of the many workers drawn to this region. It was an English-born schoolmaster, Charles Lapworth, inspired by the ideas of foreign practitioners and fascinated by the hitherto-despised graptolite, who applied a technique (zonal stratigraphy) and developed a structural paradigm (isoclinal folding) that provided an elegant and coherent solution to the myriad problems bequeathed by previous attempts.So persuasive was Lapworth's model that the Geological Survey deployed its two finest mappers, Peach and Horne, to re-examine the entire region, using Lapworth's techniques. The outcome of that 10-year task is the monumental publication commemorated here. That work, and the ideas it espoused, remained virtually unchallenged for more than half a century and contributed to the application and elaboration of then-new geotectonic ideas, such as the geosynclinal concept.However, in the 1950s the application of new or neglected techniques (way-up criteria, turbidite sedimentology, greywacke petrography, microstructural analysis, etc.) led to drastic reappraisal of prevailing structural and stratigraphic models and introduced a new paradigm (colloquially termed the ‘Southern Uplands paradox’) that envisaged a dominant role for strike-parallel major reverse faults. In contrast to Lapworth's shale-based approach this new model focussed on evidence derived predominantly from the thick intervening greywacke sequences. Investigations led by the Edinburgh and St Andrews schools extended and amplified this new interpretation of Southern Uplands geology, elucidated details of the palaeogeographic setting and the evolution of both the depositional basins and the source areas, and suggested comparisons with other parts of the Caledonian-Appalachian orogen.The advent of plate tectonics revived interest in the Southern Uplands, first as a candidate subduction-related margin, then in the late 1970s an accretionary prism origin was proposed for the Southern Uplands imbricate thrust stack. The attractions of this hypothesis were manifest and it stimulated renewed activity by academics and the Geological Survey. Significant inconsistencies and perturbations in the simple accretionary prism concept have emerged from these more detailed studies and a range of convergence zone scenarios has been proposed. A Geological Society Meeting on the topic in 1986 furnished much new data and ideas but failed to yield conceptual consensus. Thus, as yet the latest Southern Uplands Controversy remains unresolved.Changes in the level of geological research activity in the Southern Uplands can be assessed from an analysis of the numbers of relevant publications appearing over five-year intervals. This survey reveals a pattern that broadly accords with the narrative outlined above and supports the concept of two complete and one as-yet incomplete cycles of model development and adoption. Each of these developmental cycles appears to follow the progression of stages in scientific development identified by Thomas Kuhn and common to many scientific disciplines.